1. Field of the Invention
This invention relates to a magnetic recording medium and more particularly, it is concerned with an improved magnetic recording medium having excellent properties, in particular, good running property and durability.
2. Description of the Prior Art
In magnetic recording materials, in particular, audio cassette tapes, which have lately been used for music recording, an excellent frequency characteristic and sound reproducing capacity are required. With the spread of car stereo and radio-cassette system, an excellent tape running property and durability are also required. On the other hand, in video cassette tapes, recording of very high density has been carried out by shortening the recording wavelength or by narrowing the track width and to this end, a higher S/N ratio and excellent image reproducing property are required. Furthermore, with the spread of portable VTR, there is an increased demand for VTR tapes having a decreased total thickness, i.e. less than 20 .mu.m and a much higher running durability than in the prior art.
That is, both the audio tapes and video tapes are strongly required to have more excellent electromagnetic property, tape running property and durability than those of the prior art. For the purpose of meeting these requirements, various binder compositions have hitherto been proposed, but satisfactory binders have not been found yet.
At present, mixtures of copolymers of vinyl chloride/vinyl acetate/other vinyl compounds with urethane rubber resins have mainly been used as a binder composition from the standpoint of the dispersibility of ferromagnetic powders and the running property or durability of magnetic layers. The copolymers of vinyl chloride/vinyl acetate/other vinyl compounds include vinyl chloride/vinyl acetate/vinyl alcohol copolymers, vinyl chloride/vinyl acetate/maleic anhydride copolymers and the like and particularly, the vinyl chloride/vinyl acetate/maleic anhydride copolymers have often been used because of being excellent in dispersibility of ferromagnetic powders.
These prior art copolymers of vinyl chloride/vinyl acetate/maleic anhydride have generally compositions of 80 to 90% of vinyl chloride, 10 to 15% of vinyl acetate and 0.5 to 3% of maleic anhydride, which are commercially sold as a commercial name VMCH by Union Carbide Co., as S'LEC M by Sekisui Kagaku Kogyo KK, or as Denka Vinyl 1000 M by Denki Kogyo KK.
When using the prior art copolymers of vinyl chloride/vinyl acetate/maleic anhydride, however, the flexibility of a magnetic layer lacks, resulting in necessity of adding a large quantity of plasticizers and a large quantity of soft resins. In the case of adding a large quantity of plasticizers, problems tend to arise that blooming occurs and the durability of magnetic layers lowers, while in the case of jointly using a large quantity of polyurethane resins, problems tend to arise as to the dispersibility of ferromagnetic powders and the tape running property at a higher temperature and higher humidity.
Therefore, it is often difficult to obtain a magnetic recording medium with a sufficient running property and durability required at present by the prior art combinations of vinyl chloride/vinyl acetate/maleic anhydride copolymers and polyurethane resins.